Systems, methods, and a kit for determining the presence of fluids

This invention relates to nucleic-acid based product authentication and identification by determining authentication codes comprising target nucleic acids using oligonucleotide probes associated with samples. The presence of the authentication code is determined using detection methods, such as flow cytometric methods, capable of particle discrimination based on the light scattering or fluorescence properties of the particle. Target-correlated fluorescence signal, originating from a target nucleic acid hybridized to labeled complementary oligonucleotides is determined as an indicator of the presence of the authentication code. In some embodiments, an intercalating dye is used to determine the presence of target nucleotide/oligonucleotide heterodimers and identify an authentication code.